Developing clean and renewable energy sources has attracted more and more attention, due to CO2-exacerbated climate change, depletion of fossil fuels, and vehicle-related air pollution. Hydrogen is one of the most promising energy fuels for automobiles and has potential applications to smaller portable devices, like mobile phones. However, up to now hydrogen has not been used in great extent, for several technological issues. One of the main reasons limiting hydrogen usage is the difficulty of storage and delivery. To achieve a widespread usage of hydrogen as a fuel and versatile energy carrier, developing feasible chemical technologies for storage systems and large-scale hydrogen production is required.
The U.S. Department of Energy (DOE) has specified targets for hydrogen storage. The targets of the hydrogen storage capacities for year 2010 are 6 wt % and more than 0.045 kg of hydrogen per liter. Both gas compression and liquefaction of hydrogen methods are inefficient and unsuitable for the purpose of hydrogen storage since neither has met the capacity target set by the DOE.
Solid state storage of hydrogen in lattice interstitial sites is another solution that may provide high storage densities. Metals and metal alloys have been tested as hydrogen storage materials. Unfortunately, none of over 2000 known materials which can form metal hydrides satisfy all the essential requirements and have been unable to meet all of the DOE targets. Furthermore, metal hydrides are expensive and heavy, which makes them unsuitable for mobile applications. They need high desorption temperatures, and MgH2 desorbs hydrogen at a high temperature of 573 K. Ideally, desired decomposition temperature for a hydride is comparable to the waste heat of the fuel cell, which ranges from 60 to 120° C.
Gas-on-solid adsorption of hydrogen on nanostructured materials has attracted considerable attention, for a high storage density and the safe nature. Due to the considerable surface area, carbon-based nanostructures, such as graphite, graphene layer, and carbon nanotubes, appear to be suitable candidate materials for storing hydrogen.
There is considerable debate within the literature concerning whether the carbon-based nanomaterials are suitable to store a practically viable amount of hydrogen within technologically viable conditions. To date there is no agreement on this point.